Breathing is a remarkable behavior that mediates gas exchange to support metabolism and regulate pH. A reliable and robust rhythm is essential for breathing in mammals. Failure to maintain a normal breathing rhythm in humans suffering from sleep apnea, apnea of prematurity, congenital central hypoventilation syndrome, hyperventilation syndrome, Rett syndrome, and perhaps sudden infant death syndrome, leads to serious adverse health consequences, even death. Various neurodegenerative diseases, such as Parkinson's disease, multiple systems atrophy and amyotrophic lateral sclerosis, are associated with sleep disordered breathing that we hypothesize results from the loss of neurons in brain areas controlling respiration. If breathing is to be understood in normal and in pathological conditions, the underlying circuits and the functional contribution of each neuronal subtype to respiratory rhythmogenesis must be revealed. We focus on two brain sites essential for generation of the normal breathing pattern, the preBtzinger Complex and the retrotrapezoid nucleus/parafacial respiratory group. We will exploit: i) a novel method for rapid changes in excitability of genetically targeted neurons to affect respiration, and; ii) newly developed techniques for the study of neuronal projections. Using a viral delivery system, we will express genetically encoded fluorescent proteins or the allatostatin receptor in targeted subpopulations of neurons in these key regions. Rapid changes in the excitability of these neuronal subpopulations by administration of allatostatin in anesthetized or behaving rodents should produce noticeable, even profound perturbations in breathing that will illuminate their functional roles. Defining the detailed anatomical organization of the respiratory central pattern generator is essential to advance our understanding of the neural control of breathing and determining the perturbations that arise from selectively suppressing neuronal subtypes will provide a unique and extraordinary window into understanding mechanisms of central respiratory rhythm and pattern generation.